Liquid crystals may be used in a wide variety of devices, including architectural glazing products and visual display devices. The property of liquid crystals that enables them to be used in such devices is the ability of liquid crystals to transmit light on the one hand and to scatter light and/or absorb it (especially when combined with an appropriate dye) on the other, depending on whether the liquid crystals are in a relatively free, that is de-energized or field-off state, or in a relatively aligned, that is energized or field-on state. An electric field selectively applied across the liquid crystals may be used to switch between the field-off and field-on states.
There are three categories of liquid crystal material, namely cholesteric, nematic and smectic. The present invention relates in the preferred embodiment described hereinafter to the use of liquid crystal material which is operationally nematic. By "operationally nematic" is meant that, in the absence of external fields, structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal at its boundaries rather than by bulk effects, such as very strong twists (as in cholesteric material) or layering (as in smectic material). Thus, for example, a liquid crystal material including chiral ingredients which induce a tendency to twist but which cannot overcome the effects of the boundary alignment of the liquid crystal material would be considered to be operationally nematic.
A more detailed explanation of operationally nematic liquid crystal material is provided in U.S. Pat. No. 4,616,903, issued Oct. 14, 1986, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to U.S. Pat. No. 4,435,047, issued Mar. 6, 1984, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, and which disclosure is also hereby incorporated by reference.
Nematic curvilinear aligned phase (NCAP) liquid crystal material and devices using NCAP liquid crystal material are also described in the above-identified U.S. Pat. No. 4,435,047. A functional NCAP liquid crystal device may consist of NCAP film sandwiched between two electrode-coated substrates. The substrates may be polyester (PET) coated with indium tin oxide to form electrodes. The encapsulated NCAP material or film may comprise a containment medium containing plural volumes of operationally nematic liquid crystal. The plural volumes may be discrete or interconnected cavities or capsules. The interconnecting channels or passageways may also contain liquid crystal material.
A voltage source may be connected between the electrodes to selectively apply an electric field across the liquid crystal material. The liquid crystal material will scatter and/or absorb light in the field-off state and transmit light in the field-on state. Thus, the liquid crystal film may be switched between a highly translucent state (field-off) and a transparent state (field-on).
The NCAP film may be used in the construction of windows and the like. Such apparatus are described in U.S. Pat. No. 4,749,261, issued Jun. 7, 1988, entitled SHATTER-PROOF LIQUID CRYSTAL PANEL WITH INFRARED FILTERING PROPERTIES, assigned to Taliq Corporation. A window may be fabricated by laminating the electrode-coated substrate that supports the NCAP film to a window surface, for example glass or sheet plastic.
In the unpowered condition or field-off state, such windows provide privacy, glare control, shading, and daylighting by virtue of their light scattering properties. In the powered condition or field-on state, the windows are clear and provide visibility, which creates work and living spaces that are light and open, and let in views.
A problem associated with the use of encapsulated liquid crystal materials in architectural glazing products, such as windows, is that the material appears clear only over a finite viewing angle. At a normal viewing angle (0.degree. ), the material possesses minimum scattering or haze. However, the haze increases with an increase in the viewing angle, i.e., the observer moves off-axis, and this may be objectionable as it tends to obscure images when it is desired that maximum clarity be provided in the powered state.
This effect is caused by the mismatch of the refractive indices of the containment medium and the liquid crystal material. Liquid crystal materials useful in such applications have anisotropic refractive indices, with the refractive index along the long molecular axis typically higher than along either of the two short molecular axes. The refractive index along the long molecular axis is called the extraordinary index of refraction, while the refractive index along the short molecular axis is called the ordinary index of refraction.
If the ordinary refractive index of the liquid crystal material is matched to the refractive index of the containment medium, light incident on the material at a normal angle provides minimum scattering in the field-on state, since the match of the refractive indices of the liquid crystal material and the containment medium is optimized. At increased viewing angles, however, for one polarization of incident light, the effective refractive index of the liquid crystal material is higher than its ordinary refractive index; that is, the effective refractive index of the liquid crystal material appears to be an admixture of its ordinary and extraordinary indices of refraction. This results in increased scattering and thus haze as the viewing angle moves away from normal.
This haze in the field-on state is also higher, at any viewing angle and RMS voltage, if the film is powered by a sine-wave power source rather than by a square wave power source. As is known, a sixty cycle sine-wave power source is an alternating current source wherein the applied voltage switches from a positive to a negative voltage (and then back) sixty times a second. As a result, the applied voltage on the film necessarily is at zero voltage one-hundred and twenty times a second for short periods of time. Since encapsulated films may have very fast response times, the film turns partially off during these zero crossing periods. Since the human eye cannot follow such a rapid response, it averages such decay behavior, and as a result the film appears to be hazy. This increased haze is objectionable.
In view of the above, a square wave power supply is typically utilized to power encapsulated liquid crystal materials. While such a power source also crosses zero volts one-hundred twenty times a second, it does so much more rapidly than the sine-wave power source. A square wave power supply reverses polarity almost instantaneously. Thus, the liquid crystal material does not have time to switch between a distorted and aligned state. Therefore, the match between the ordinary refractive index of the liquid crystal and the refractive index of the containment medium is maintained during each cycle. As such, additional haze is not caused by application of the electrical field. However, the use of a square wave power supply adds cost and complexity to any product, such as architectural glazing products, incorporating an encapsulated material.
Accordingly, an object of the present invention is to provide an encapsulated liquid crystal material that has low haze over a wide viewing angle.
Another object of the present invention is to provide an encapsulated liquid crystal material that may be powered by a sine-wave power source and has low haze over a wide viewing angle.